A power converter for driving a three phase motor, for example, is configured by a half-bridge circuit provided for each of the three phases connected in parallel between positive and negative DC (Direct Current) source lines. The half-bridge circuit comprises a pair of semiconductor switches series connected between DC power source lines and a freewheeling diode in an inverse-parallel connection with each of the semiconductor switches. In such power converter, the semiconductor switches are driven by PWM (Pulse Width Modulation) control and thus, DC power given by the DC power source line is converted into three-phase AC power to energize the motor windings with a sinusoidal current.
Under such configuration, improvement in controllability, reduction of audible energizing sound originating from PWM modulation, and downsizing of peripheral components is being sought through increased PWM frequency.
In PWM control, shorting between DC power source lines are prevented through the half-bridge circuit by providing a so-called dead time during which the pair of semiconductor switches are both turned OFF. Increasing the PWM frequency increases the duration simultaneous OFF period within a PWM period. Thus, sufficient ON time needs to be obtained by reducing or accelerating the turn-on time (rise time) of the semiconductor switches.
Given such circumstances, a motor drive system employing the power converter configured as described above is facing increase in common-mode current flowing into the earth due to noise originating from sudden voltage variation at the coil neutral point. Floating capacitance, which is a parasitic capacitive component, is found on various parts of the motor such as the coil, stator, rotor, housing, and the rotary shaft. When the motor is used in in-vehicle system applications such as in electric vehicles or the like, the floating capacitance becomes capacitively coupled to the metal chassis. As a result, common-mode current flows throughout the chassis by way of the capacitively coupled component to increase the common-mode noise.
A typical approach for suppressing common-mode noise is providing dedicated additional circuitry such as a common-mode transformer and a common-mode-current prevention circuit which tend to be sizable. Such approach increases the complexity of the circuit as well as the overall size and manufacturing cost. Various other approaches for reducing the common-mode noise have been conceived other than those described above. However, neither of such approaches provides an easy solution for common-mode noise originating from high-frequency variation of surge voltage produced by shorting-circuit current flowing between the DC power source lines at the end of dead time. The short-circuit current is caused by the reverse current flow (recovery current) produced by the transport of remaining carrier when the freewheeling diode goes through reverse recovery after freewheeling current flows the freewheeling diode during dead time.